(a) Industrial Field of the Invention
The present invention relates to an electrophotographic photoreceptor. More specifically, the present invention relates to an electrophotographic photoreceptor which due to its ability of maintaining its superior mechanical strength and outstanding electrophotographic properties, i.e., characteristics required in electrophotography, over a long period, can be suitably utilized in a variety of application fields of electrophotography.
(b) Description of the Related Art
In recent fields of electrophotography, the main current of photoreceptors has been layered-type of organic electrophotographic photoreceptors of which the photosensitive layer contains at least two elementary layers, a charge generation layer where charges are generated by exposure and a charge transport layer where transport of the potential occurs. In this sort of layered-type organic photoreceptors, a binder-resin is used as a component of its charge transport layer and a polycarbonate resin obtained by using bisphenol-A as a starting monomer has been widely used as the binder-resin.
Polycarbonate resins made from bisphenol-A generally have such advantageous characteristics that because of their good compatibility with charge transporting materials, they render the resulting photoreceptors good electrical properties and high mechanical strength.
However, it has been found that problems, including the following problems (1)-(3), arise in the case that the charge transport layer of a photoreceptor is formed by using a polycarbonate resin made from bisphenol-A as the binder-resin.
(1) In preparation of a photoreceptor, whitening (gelatin) of a coating solution applied for forming the charge transport layer tends to occur depending on the kind of the solvent used for preparing the coating solution to be applied, and the formed charge transport layer tends to crystallize easily. This crystallization causes quality defects of the developed image since photo-induced discharge hardly occurs on the crystallized regions of the charge transport layer, leaving the residual charges which cause an undesirable electric potential on the regions.
(2) Solvent-crack of the polycarbonate resin derived from bisphenol-A is often caused in the charge transport layer by another solvent used for applying another layer onto the charge transport layer. In other words, exposing the once prepared charge transport layer with another solvent greatly decreases the mechanical strength of the charge transport layer. If the resulting photoreceptor is used by rotating for long time in a copying machine, the charge transport layer will get cracked causing the appearance of some crack-patterns on the resulting copied articles.
(3) A charge transport layer formed by using a polycarbonate resin made from bisphenol-A as its binder-resin tends to be peeled off from the base layer because of its poor adhesion to the base layer. Consequently, the resulting photoreceptor often meets such a disadvantage that its life in practical use for copying is short. The base layer described above usually indicates a charge generation layer. However, it may indicate an electric conductor in the case of a positively-charged-type electrophotographic photoreceptor where a charge transport layer and a charge generation layer are successively laminated on an electric conductor in that order, or it may be an intermediate layer (such as a blocking layer, etc.) in the case that the intermediate layer is formed between an electric conductor and a charge transport layer or between a charge generation layer and a charge transport layer with aiming at improving electrophotographic properties.